Implantable medical devices (IMDs), such as cardiac pacemakers and implantable cardioverter defibrillators (ICDs), provide therapeutic electrical stimulation to a heart of a patient via electrodes carried by one or more medical electrical leads and/or electrodes on a housing of the medical device. The electrical stimulation may include signals such as pacing pulses or cardioversion or defibrillation shocks. In some cases, a medical device may sense cardiac electrical signals attendant to the depolarizations of the heart and control delivery of stimulation signals to the heart based on sensed cardiac electrical signals. Upon detection of an abnormal rhythm, such as bradycardia, tachycardia or fibrillation, an appropriate electrical stimulation signal or signals may be delivered to restore or maintain a more normal rhythm of the heart.
Single chamber pacemakers sense cardiac electrical signals in a single heart chamber and deliver pacing pulses to the heart chamber in the absence of electrical activity. Dual chamber pacemakers sense cardiac electrical signals in two heart chambers, e.g., in the atrial and ventricular chambers, and may deliver cardiac pacing pulses in one or both chambers to provide appropriate timing and synchrony between the contractions of the atrial and ventricular chambers.